As is known, electro-optical modulators are crucial optical components for optical digital and analog links for signal transmission and processing, in that they are capable of modulating optical power of optical carriers by means of driving electrical signals.
EOSSBMs are especially useful in optical fiber communication systems, such as higher density wavelength multiplexing and long-haul fiber transmission due to reduced non-linear optical effects, because of reduced optical power.
The presence of a single side lobe in the optical spectrum of a SSB-modulated optical signal results in several technical advantages, the main of which are half spectral occupation compared to standard Double SideBand (DSB) modulation and no fading of the optical signal at the end of an optical link when used fiber optics are dispersive.
On the other hand, EOSSBMs are usually quite complex in their internal configuration, in their operation, and in the external drivers needed for their operation.
A known Mach-Zehnder (MZ)-type EOSSBM is disclosed in Higuma, K., Hashimoto, Y., Nagata, H., Oikawa, S., Izutsu, M., “X-cut LiNbO3 optical SSB modulators”, Electronics Letters Vol. 37 (2001), pp. 515-516 and schematically depicted in FIG. 1.
The EOSSBM is formed in an LiNbO3, X-Cut, Y-propagation substrate and includes a primary, nested MZ structure made up of two optical, Ti-diffused, MZ waveguide paths each including a sub-MZ structure MZA, MZB. Two RF modulation ports RFA and RFB and three DC bias ports DCA, DCB and DCC are provided.
A DC supply is supplied to the sub-structure MZA from bias port DCA to result in a n phase shift between the two sub-arms in sub-structure MZA, to sub-structure MZB from bias port DCB to result in a n phase shift between the two sub-arms in sub-structure MZB, and to primary structure MZ from bias port DCC to result in a π/2 phase shift between sub-structures MZA and MZB. An RF modulation signal Φ1(t)=Φ·cos Ωt is supplied to sub-structure MZA from modulation port RFA, and an RF modulation signal Φ2(t)=Φ·sin Ωt is supplied to sub-structure MZB from modulation port RFB and generated by a wideband 90-degrees phase shifter inputted with Φ1(t).
Despite its undeniable performance in terms of carrier suppression ratio, it is equally undeniable that this EOSSBM is everything but simple: in fact, two levels of nested MZ modulators in the optical pattern are provided, three DC bias signals are needed during operation with proper electronic stabilization loops, and two different, π/2 shifted, RF driving signals are needed.
A more standard configuration of a known MZ-type EOSSBM is disclosed in Smith, G. H., Novak, D., Ahmed, Z., “Technique for optical SSB generation to overcome dispersion penalties in fibre-radio systems” Electronics Letters Vol. 33 (1997), pp. 74-75 and schematically depicted in FIG. 2.
Despite a single MZ modulator is featured, a proper DC bias point stabilization together with wideband processing of the RF modulating signal is still needed, which includes signal splitting and π/2 phase shifting of one of the two split signals.
U.S. Pat. No. 5,022,731 discloses (FIG. 12) a frequency shifter which can be employed in a EOSSBM device operable to transmit a signal V(t) by heterodyning between two optical signals. To this end, a laser source is employed, which emits a substantially monochromatic optical signal at an optical frequency. The optical signal is divided into two in order to excite two input optical fibers in equal manner, wherein a first one of the input optical fibers is connected to a frequency shifter which receives an electric signal V(t)ej2πft. The optical frequency-shifted signal is transferred to an output optical fiber. The optical signal from the laser source and supplied to the second one of the two input optical fiber is combined to the frequency-shifted optical signal. From this point on, the optical spectrum of the resulting optical signal is that of an SSB-modulated optical signal. Nevertheless, this approach to implement SSB modulation clearly looks quite complex in term of optical circuit with two nested MZ-like structures, the one made up with optical fibers also being potentially affected by large thermo-mechanical instability and fluctuations. Also a proper DC bias point stabilization is still needed.
In this architecture, as well as in the others mentioned above, at each recombination Y junction a partial destructive interference effect occurs, thus causing part of optical power being lost in radiative modes.